1. Field of the Invention
This invention relates to the general subject of seismic exploration and, in particular, to methods for displaying attributes of a 3-D volume of seismic data for identifying structural and stratigraphic features in three dimensions.
2. Background of the Invention
2-D seismic data is acquired along lines that consist of geophone arrays onshore or hydrophone streamer traverses offshore. FIG. 1, shows an example of portions of a marine seismic data acquisition system. A vessel 10 on a body of water 15 overlying the earth 16 has deployed behind it a seismic source array 20 and a streamer cable 25. The seismic source array 20 is typically made up of individual air guns (not shown) that are fired under the control a controller (not shown) aboard the vessel 10. Seismic pulses propagate into the earth and are reflected by a reflector 22 therein. Exemplary raypaths 41a, 41b from the source to the receiver are shown. For simplifying the illustration, only one reflector is shown: in reality, there would be numerous reflectors, each giving rise to a reflected pulse. After reflection, these pulses travel back to the surface where they are recorded by detectors (hydrophones) 30a, 30b, . . . 30n in the streamer cable. The depth of the source array and the streamer cable are controlled by auxiliary devices (not shown). In acquiring a line of seismic data, the vessel 10 travels in the water and periodically fires the airgun 20 at different source locations. Data corresponding to each such source location are recorded by the plurality of receivers.
The acquisition geometry for a full 3-D data set on land is illustrated in FIG. 2 wherein, within a region 119, sources 124 are deployed along a plurality of source lines 126a, 126b . . . 126n and data are recorded by receivers 122 along receiver lines 120a, 120b . . . 120n nominally defining an inline direction. In conventional processing, data from the plurality of sources and receivers are output into bins such as 121. With this high density coverage, extremely large volumes of digital data need to be recorded, stored and processed before final interpretation can be made. Processing requires extensive computer resources and complex software to enhance the signal received from the subsurface and to mute accompanying noise which masks the signal.
3-D marine seismic data may be acquired (not shown) by using a plurality of widely spaced parallel streamers recording energy that has been generated by a number of seismic sources that are spaced apart in the crossline direction.
Once the data is processed, geophysical staff compile and interpret the 3-D seismic information in the form of a 3-D cube which effectively represents a display of subsurface features. Using the data cube, information can be displayed in various forms. A commonly used display comprises horizontal time slice maps can at selected depths, an example of which is shown in FIG. 3. Using a computer workstation an interpreter can slice through the field to investigate reservoir issues at different horizons. Vertical slices or sections can also be made in any direction using seismic or well data. Time maps can be converted to depth to provide a structural interpretation at a specific level.
Seismic data has been traditionally acquired and processed for the purpose of imaging seismic reflections. Changes in stratigraphy are often difficult to detect on traditional seismic displays due to the limited amount of information that stratigraphic features present in a cross-section view. Although such views provide an opportunity to see a much larger portion of these features, it is difficult to identify fault surfaces within a 3-D volume where no fault reflections have been recorded.
U.S. Pat. No. 5,563,949 to Bahorick et al teaches dividing the three-dimensional volume into a plurality of vertically stacked and generally spaced apart horizontal slices; dividing each of the slices into a plurality of cells; measuring across each of the cells the cross-correlation between one pair of traces lying in one vertical plane to obtain an inline value and measuring the cross-correlation between another pair of traces lying in another vertical plane to obtain a crossline value that are estimates of the time dip in an inline direction and in a crossline direction; combining the inline value and the crossline value to obtain one coherency value for each of the cells; and displaying the coherency values of the cells across. Such a coherency display is particularly well suited for interpreting fault planes within a 3-D seismic volume and for detecting subtle stratigraphic features in 3-D. This is because seismic traces cut by a fault line generally have a different seismic character than traces on either side of the fault. Measuring trace similarity, (i.e., coherence or 3-D continuity) along a time slice reveals lineaments of low coherence along these fault lines. Such coherency values can reveal critical subsurface details that are not readily apparent on traditional seismic sections. Also by calculating coherence along a series of time slices, these fault lineaments identify fault planes or surfaces.
U.S. Pat. No. 5,892,732 to Gersztenkorn discloses a modification of the Bahorich invention wherein a covariance matrix is determined for each of the cells and a seismic attribute determined from the eigenvalues of the covariance matrix is displayed. Gersztenkorn teaches that the ratio of the dominant eigenvalue of the covariance matrix to the sum of the eigenvalues is an indication of the coherence of the data.
U.S. Pat. No. 6,055,482 to Sudhakar et al. teaches display of other types of seismic attributes in a 3-D data volume. For example, azimuth ordered seismic gathers are used to identify subterranean features such as fault and fracture patterns. Offset ordered coherence analysis is used to form an optimum stack at the subterranean location of interest.
A number of prior patents teach the use of color for displaying of seismic data to bring out features that are normally lost in a conventional seismic display. The teachings of U.S. Pat. No. 4,467,461 to Rice allow the interpreter to more easily comprehend simultaneous variation of several geophysical data attributes and to relate the effects to a specific end result for the geophysical indicators of interest. One or more geophysical attribute variables are quantified and then rasterized so that the data is represented as a gridded variable area display wherein color intensity of the grid units is some function of the instantaneous variable. The resulting data is then loaded into digital refresh memory of an image processing computer whereupon it us interactively mixed for analysis in accordance with operator selected colors and color intensity weighting.
In U.S. Pat. No. 5,995,448 to Krehbiel, a suite of features extracted from a sequence of windows form a multivariate attribute of the raw data. These features include the energy, slope in the middle of a window, the autocorrelation, average trace amplitude, standard deviation of the amplitude, first and second lags of the autocorrelation. Combinations of three of these features are color coded and superimposed on a display of the seismic section.
U.S. Pat. No. 5,930,730 to Marfurt et al teaches the use of color displays for a 3-D volume of seismic data. A color map, characterized by hue, saturation and lightness, is used to depict semblance/similarity, true dip azimuth and true dip of each cell; true dip azimuth is mapped onto the hue scale, true dip is mapped onto the saturation scale, and the largest measurement of semblance/similarity is mapped onto the lightness scale of the color map.
PCT Patent Publication WO 0014574 to Giertsen et al discloses a method of producing one or more volume windows within a 3-D data volume that can be interactively moved around in the entire data volume and viewed from different positions at different angles. By color and opacity manipulations inside the volume windows, the data therein are made transparent, allowing for visualization of selected target portions of the data set.
As pointed out in Giertsen, it is difficult to get a good understanding of complicated 3-D data set on a flat screen or a piece of paper. It is also laborious and difficult to interact with 3-D objects using a keyboard and a mouse. Analysis of 3-D data sets are commonly done using 2-D slices through the data cube. Furthermore, the screen size limits the amount of information that can be presented. Yet another drawback of prior methods of displaying 3-D data is that the interactive output of a graphical workstation is necessarily viewable only by a limited number of viewers. The present invention addresses these problems.
The present invention is a method of geophysical prospecting wherein a set of seismic traces are obtained over a predetermined three-dimensional (3-D) volume of the earth. At least three substantially parallel surfaces are defined in the 3-D volume and a different color is associated with each of the surfaces. Values of a seismic attribute on the three surfaces are encoded onto corresponding color scales and displayed in a single display. In one embodiment of invention, the selected colors are red, green and blue. In a preferred embodiment of the invention, the seismic attribute is the amplitude of the trace, although any of a commonly used set of seismic attributes may be displayed. The three surfaces may be isotime surfaces or they may be parallel to a fault plane or to an interpreted horizon. With this type of display, it is possible to visualize stratigraphic sequences along with the overall geometry of subsurface geological features.